This invention in general relates to optical communications and in particular to wavelength selective devices for transferring optical signals, or portions of optical signals, from one optical fiber line to another.
The use of optical fiber as a medium for transmitting information as modulated electromagnetic waves in the optical region of the spectrum is a well-established practice known throughout the communications industry. The motivation for using optical fiber stems in part from its enormous information carrying capacity which, for a variety of reasons, has not yet been fully exploited. Major reasons for this are the lack of ability to modulate signal sources at rates comparable to the optical fiber carrying capacities and the inability of detectors to respond at rates comparable to the maximums allowable by the fiber. Even so, optical fiber based communication systems are still favored, particularly for future applications, and those skilled in the art have recognized that meanwhile fiber capacity can be exploited in ways other than through increases in source and detection modulation rates. For example, the total information per fiber goes up in proportion to the number of channels carried. Advantageously, the number of switching ports also increases as the number of channels increases, and the cost per channel decreases. Thus, through the simple expediency of increasing the number of channels carried per fiber, one can increase the total information transferred even at current source modulation and detection rates.
However, for optimal results, this approach is best with narrow spacing between wavelengths assigned channels and requires wavelength selective coupling devices capable of separating off channels where required in a network as at switching ports, branches, repeaters, demultiplexing stations, or subscriber ports.
Wavelength selective couplers are known in the art. One coupler uses a dichroic reflector in line to achieve full duplex operation. More than two channels can be multiplexed using this technique with more than one dichroic if different in characteristic.
Other couplers employ prism dispersion or diffraction gratings to spatially separate different channels and then focus the spatially separated channels with appropriate bulk optics into preselected fibers for further use.
Wavelength selective lateral coupling between optical fiber devices, as opposed to in-line coupling, is also possible. U.S. Pat. No. 4,342,499 issued Aug. 3, 1982, in the name of John W. Hicks, Jr., for example, describes dispersive lateral coupling between two fiber cores. However, the construction of couplers by such means requires precise control of fiber diameter and relatively long coupling lengths between cores for very narrow bandwidth transfer.
Another means for wavelength selective lateral coupling is described in U.S. patent application Ser. No. 331,052 filed in the name of John W. Hicks, Jr. on Dec. 16, 1981, now abandoned, and entitled "OPTICAL RESONANT CAVITY FILTERS". Here, either polished and highly reflective metallic mirrored ends or ends with highly reflective multilayer interference coatings are required to define an optical resonant cavity which is, in turn, laterally coupled into a trunk line. At the scale required, it appears relatively difficult to fabricate such reflective ends.
Consequently, it is a primary object of the present invention to provide a wavelength selective coupler for transferring closely spaced signals between optical fibers.
Another object of the present invention is to provide a coupler which is wavelength selective and yet relatively easy to fabricate.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the detailed disclosure which follows.